This invention relates to cartridges for firearms that operate, or are converted to operate, on the blow-back principle. It is particularly suited for use as training ammunition for firing low mass training projectiles, such as that described in U.S. Pat. No. 5,035,183.
A number of telescopically expanding cartridge cases have been developed to facilitate the operation of blow-back cycling firearms. Such rounds are needed when a low mass projectile is being fired as the recoil in such cases is greatly diminished.
Examples of such prior art rounds are described in U.S. Pat. Nos. 5,359,937 and 5,016,536.
An objective in the designing of such rounds is the minimization of cost. Various materials including plastics and metals may be employed. A further factor for consideration is that a training round which is intended to operate in a minimally converted, standard, firearm should chamber and eject reliably, in the same manner as a normal round. For these and other reasons it is desirable to rely on metals. However, it is difficult to achieve high sealing efficiencies between sliding metal surfaces.
In the case of rounds with telescopically expanding casings, a tight sliding fit is required between parts to contain propellant gases during the cartridge expansion action. This type of fit is difficult to achieve with all-metal parts, and even when non-metal materials are employed.
The present invention addresses the problem of providing a seal under such circumstances.
The invention in its general form will first be described, and then its implementation in terms of specific embodiments will be detailed with reference to the drawings following hereafter. These embodiments are intended to demonstrate the principle of the invention, and the manner of its implementation. The invention in its broadest and more specific forms will then be further described, and defined, in each of the individual claims which conclude this Specification.
According to the invention in one aspect a telescopically expanding cartridge equipped with a projectile or formatted as a blank comprises a two part casing portion, which portions are telescopically interfitted into each other over an overlapping region. One xe2x80x9chead endxe2x80x9d portion of the casing carries the head end of the cartridge and the other, forward, portion constitutes the chambering end portion of the casing, hereafter referred to as the xe2x80x9cplugxe2x80x9d.
When assembled, the rearward end of the plug fits, in one variant with a close sliding fit, into a sleeve that is part of the head end portion of the casing. The plug rearward end terminates in a transverse wall pierced by an orifice. An interior passage extends axially from this orifice to the forward end of the casing, providing a path for propellant gases to escape through the orifice and propel a projectile, if present. The transverse wall and inside surfaces of the sleeve and head end delimit an internal volume or cavity within the casing.
Fitted against the transverse wall within the internal volume is a sealing disc of flexible, compliant, preferably polymeric material. This disc is dimensioned to effect a tight sliding fit, preferably an interference fits against the inner wall of the sleeve. It contains a central hole aligned with the orifice in the transverse wall of the plug. This central hole may function as a xe2x80x9cchoke holexe2x80x9d to allow gases evolving in the inner cavity to escape in a metered manner from the cavity, out through the end wall orifice. Alternately, a choke hole may be formed in the transverse wall as the end wall orifice, accessed through a simple hole in the disc.
Optionally but preferably a frangible membrane overlies the choke hole to seal-out moisture before the firing of the round. Ignition of propellant within the inner cavity ruptures this membrane.
On firing, gases evolving in the inner cavity develop an over-pressure that causes the casing to expand telescopically. The overlapping portion of the plug slides within the sleeve as the gap between the transverse wall and head end increases. Pressure is applied to the plug through the sealing disc which travels forwardly with the plug with respect to the sleeve. Throughout its travel, the sealing disc minimizes the loss of gases through the plug/sleeve interface.
The invention is particularly suited to the situation where the sleeve is made of brass and the plug is made of zinc, or a zinc alloy.
To position the sealing disc accurately, according to one variant, the transverse wall may have a recess or protrusion, preferably circular, and the disc may have a correspondingly shaped.
The rearward end of the plug need not be in direct contact with the inside surface of the sleeve to provide a close sliding fit.
The sealing disc may also or alternately have a thin, axial extending rim located on the outer periphery of the disc and embracing the outer perimeter of the rearward end of the plug to seal against the interior wall passageway of the sleeve. This rim is thrust against the sleeve walls under the gas pressure developed by the propellant upon firing, increasing the sealing properties of the disc. This rim can also provide an alternate or supplementary means for positioning the disc centrally on the rearward end of the plug.
To assist in assembly, this rim may have a hooked edge that lockingly engages with a circumferential flange formed at the rearward end of the plug. The disc may be made of a resilient material to permit the disc to be pressed into position on the rearward end with a xe2x80x9csnapxe2x80x9d fit.
The foregoing summarizes the principal features of the invention and some of its optional aspects. The invention may be further understood by the description of the preferred embodiments, in conjunction with the drawings, which now follow.